Pyrazole based NLOphores: Synthesis, photophysical, DFT, TDDFT studies

Article abstract of DOI:10.1016/j.dyepig.2015.12.026

A series of pyrazole based D-π-A derivatives have been synthesized from 3-(4-nitrophenyl)-1-pheny-1H-pyrazole-3-carbaldehyde with series of active methylene compounds. The dyes were well characterized by FT-IR, ¹H NMR, ¹³C NMR, eleme

Full text of DOI:10.1016/j.dyepig.2015.12.026

Accepted Manuscript
Pyrazole based NLOphores: Synthesis, Photophysical, DFT and TDDFT studies
Sandip K. Lanke, Nagaiyan Sekar
PII:
S0143-7208(15)00513-6
10.1016/j.dyepig.2015.12.026
DYPI 5049
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 5 November 2015
Revised Date: 7 December 2015
Accepted Date: 22 December 2015
Please cite this article as: Lanke SK, Sekar N, Pyrazole based NLOphores: Synthesis, Photophysical,
DFT and TDDFT studies, Dyes and Pigments (2016), doi: 10.1016/j.dyepig.2015.12.026.
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ACCEPTED MANUSCRIPT
Pyrazole based NLOphores: Synthesis, Photophysical, DFT, TDDFT
studies
Sandip K. Lanke, Nagaiyan Sekar*
Department of Dyestuff Technology, Institute of Chemical Technology, (Formerly UDCT)
Nathalal Parekh Marg, Matunga, Mumbai - 400 019. (India)
*Corresponding author. Tel.: +91 22 3361 1111/2222, 2707(direct),
Fax: +91 22 3361 1020. E-mail: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com
Graphical Abstract

ACCEPTED MANUSCRIPT
Pyrazole based NLOphores: Synthesis, Photophysical, DFT and TDDFT
studies
Sandip K. Lanke, Nagaiyan Sekar*
Department of Dyestuff Technology, Institute of Chemical Technology, (Formerly UDCT)
Nathalal Parekh Marg, Matunga, Mumbai - 400 019. (India)
*Corresponding author. Tel.: +91 22 3361 1111/2222, 2707(direct),
Fax: +91 22 3361 1020. E-mail: n.sekar@ictmumbai.edu.in, nethi.sekar@gmail.com
Abstract
A series of pyrazole based D-π-A derivatives have been synthesized from 3-(4-nitrophenyl)-
1-pheny-1H-pyrazole-3-carbaldehyde with series of active methylene compounds. The dyes
1
were well characterized by FT-IR, H NMR, 13C NMR, elemental analysis and mass
spectroscopy. The synthesized dyes absorb in the range of 351 to 432 nm. Experimental
absorption wavelengths for compounds are in good agreement with those predicted using the
Time-Dependent Density Functional Theory (TD-DFT) [B3LYP/6-31G(d)]. Density
Functional Theory calculations were performed to determine the static first
hyperpolarizability (β0) and related properties (µ, α, ∆α, β0 and γ) for pyrazole based “donor-
π-acceptor” dyes 3a-3f, using B3LYP functional with 6-31G (d) basis sets at the ground state.
The computed values for these dyes show high first order hyperpolarizability in the range
50.64 – 256.38 x 10-30 esu and second hyperpolarizability (γ) in the range of 235.71 to
1580.81 x 10-36 esu.
Keywords: Pyrazole, Synthesis, DFT, TDDFT, NLO study.
1. Introduction:
Pyrazole based dyes find applications in medicinal chemistry, agrochemicals and dye
chemistry. Pyrazole has played a crucial role in the development of fused heterocyclic ring
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systems, building block in biological and pharmacological intermediates [1–4], and they are
widely used as intermediates in the dyestuff industry [5,6]. Pyrazole ring can act as an
electron donor in a chromophoric system. The pyrazole ring has an electron-rich nitrogen
atom which involves in conjugation by donating electron into the chromogenic molecule.
Pyrazole containing molecules are known to show good electron-donating capacity and high
hole transfer efficiency [7]. Pyrazole derivatives have been used as hole-transport materials in
the electrophotography and electroluminescence fields [8]. Pyrazole derivatives are well
known as blue emitters and can be used as blue electroluminescent materials in organic light-
emitting diodes (OLEDs) [9,10]. p-Aryl-substituted pyrazole derivatives have also attracted
considerable interest due to their wide range of applications in DSSCs [11], as well as second
order nonlinear optical devices [12].
In recent decades, variety of chromophore have been studied as NLO materials. But several
studies have demonstrated that donor chromophores bearing additional heteroatoms could
dramatically increase the conjugation of the π-electron networks and the intramolecular
charge transfer properties which could further influence the chromophores’ first-order
hyperpolarizability. The optical properties and NLO response can be tailored by i) by
increasing λmax, as increase in absorption wavelength leads to decrease in transition energy.
For example near infrared active dyes are showing high hyperpolarizability values [13]. ii)
Increase in transition dipole moment and increase in solvent polarity are also responsible for
enhancing molecular hyperpolarizability [14,15]. iii) Molecules having twisted
intramolecular charge transfer properties exhibits high values of hyperpolarizability
[13,16,17]. iv) The β value might be properly influenced by adjusting the HOMO−LUMO
energy gap of organic molecules [18–22].
In view of these findings, and in continuation of our previous studies [23,24] we have
synthesized pyrazole based NLOphores. To synthesize pyrazole based dyes, a variety of
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active methylene compounds were used, such as malononitrile, methyl cyanoactetate, 2-
(3,5,5-trimethylcyclohex-2-enylidene)malononitrile, cyanomethylbenzthiazole, 3-methyl-1-
phenyl-1H-pyrazol-5(4H)-one and 2-(1-phenylethylidine) malononitrile. We have developed
wide range of dyes which absorb from blue to yellow region. The newly synthesized
derivatives of pyrazole contains electron rich pyrazole unit and strong acceptor group
separated by π-conjugated bonds exhibiting high values of dipole moment and
hyperpolarizability, which are desired properties for nonlinear optical materials.
2. Experimental Section
2.1. Computational Strategy
All the computations were performed on a HP workstation XW 8600 with Xeon processor, 4
GB RAM and Windows Vista as operating system. The software package used was Gaussian
09 [25]. DFT method with B3LYP functional and 6-31G(d) basis set was used for the ground
state optimization. The hybrid functional namely B3LYP (Becke3-Lee-Yang-Parr hybrid
functional) [26–29] was used. The 6-31G(d) basis set was used for all the atoms and later was
ascertained in the literature [30]. The Polarizable Continuum Model (PCM) [31] was used to
optimize the ground state geometries. The solvents used were toluene, tetrahydrofuran (THF),
1,4-dioxane (dioxane), chloroform (CHCl3), dichloromethane (DCM), ethyl acetate (EtOAc),
ethanol (EtOH), methanol (MeOH), N,N-dimethyl formamide (DMF), dimethyl sulfoxide
(DMSO). The excitation energies, oscillator strengths and orbital contribution for the lowest
20 singlet-singlet transitions at the optimized geometry in the ground state were obtained by
TD-DFT calculations using the same basis set as for the geometry minimization in solvent
environments.
2.2. Materials and Methods
1-(4-Nitrophenyl)ethanone, phenyl hydrazine, N,N-dimethylformamide, phosphorus
oxychloride, sodium hydroxide and conc. H2SO4 were purchased from S. D. Fine Chemicals
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Ltd, Mumbai, India and Sigma Aldrich. Solid reagents were characterized by melting point
and used without further purification. Liquid reagents distilled at their boiling points and used
thereafter. Synthetic grade solvent were used for synthesis and purification. Solvents were
used after distillation at their boiling point and drying according to standard processes. All
the reactions were monitored on precoated silica gel aluminium based plates kisel gel 60
F254 Merck, India. Purification of all the compounds were achieved by recrystallization and
column chromatography whenever necessary. Melting points are uncorrected and were
recorded on instrument from Sunder Industrial Product Mumbai. The absorption spectra of
the compounds were recorded on a Perkin- Elmer Lamda 25 UV-visible spectrophotometer.
The FT-IR spectra were recorded on a Jasco 4100 Fourier Transform IR instrument (ATR
1
accessories). H NMR and 13C NMR spectra were recorded on a VARIEAN Inc. USA 500
MHz instrument. Chemical shifts (δ) are reported relative to tetramethylsilane (δ = 0.0) as an
internal standard. Mass spectra were recorded on Finnigan Mass spectrometer.
2.3. Synthesis and Characterization
2.3.1. Synthesis of (E)-1-(1-4-nitrophenyl)ethylidene)-2-phenylhydrazine (1) & 3-(4-
nitrophenyl)-1-phenyl-1H-pyrazole-4-carbaldehyde (2)
The compound 1 was prepared by condensation of p-nitroacetophenone and phenyl hydrazine
in ethanol at reflux temperature using the reported procedure [32]. Compound 1 was further
subjected to Vilsmeier-Haack reaction using DMF/POCl3 to afford 3-(4-nitrophenyl)-1-
phenyl-1H-pyrazole-4-carbaldehyde (2) in good yield according to the procedure has been
described by Kira et al. [33].
2.3.2. 2-((3-(4-nitrophenyl)-1-phenyl-1H-pyrazol-4-yl)methylene)malononitrile (3a)
3-(4-Nitrophenyl)-1-phenyl-1H-pyrazole-4-carbaldehyde (293 mg,
1
mmol) and
malononitrile (66 mg, 1 mmol) were dissolved in dry methanol (15 mL). 0.1 mL of piperidine
were added, and the solution was stirred at RT for 2 h. After completion of the reaction, the
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off white solid was filtered, washed with methanol, and dried. The dye obtained was purified
by column chromatography using silica gel 100-200 mesh and toluene as eluent system.
Color: Off white solid, Yield: 286 mg, 84%, M. P.: 196 – 198 oC.
FT-IR (KBr, cm-1): 2224 (cyano stretching), 1528 (N-O), 1346 (N-O)
1H NMR (500 MHz, CDCl3) δ 7.48 (t, J = 7.4 Hz, 1H), 7.57 (t, J = 7.8 Hz, 2H), 7.76 (s, 1H),
7.77-7.86 (dd, J = 10.4, 8.6 Hz, 4H), 8.42 (d, J = 8.6 Hz, 2H), 9.10 (s, 1H).
13C NMR (126 MHz, CDCl3) δ 80.2, 113.5, 115.0, 120.1, 124.4, 129.1, 129.7, 129.9, 130.0,
136.5, 138.3, 148.5, 149.7, 153.6.
MS (m/z): (M+H) Calcd for C19H12N5O2: 342.09 found 342.10.
Anal. Calcd. for C19H11N5O2: C, 66.86; H, 3.25; N, 20.52. Found: C, 66.80; H, 3.24; N,
20.44.
2.3.3. (E)-Methyl 2-cyano-3-(4-nitrophenyl)-1-phenyl-1H-pyrazol-4-yl)acrylate (3b)
3-(4-nitrophenyl)-1-phenyl-1H-pyrazole-4-carbaldehyde (293 mg, 1 mmol) and methyl-2-
cyanoacetate (100 mg, 1 mmol) were dissolved in dry methanol (15 mL). 0.1 mL of
piperidine were added, and the solution was stirred at RT for 2 h. After completion of the
reaction, the off white solid was filtered, washed with methanol, and dried. The dye obtained
was purified by column chromatography using silica gel 100-200 mesh and toluene as eluent.
Color: Off white solid, Yield: 314 mg, 84%, M. P.: 188 – 189 oC;
FT-IR (KBr, cm-1): 2221 (cyano), 1726 (ester), 1526 (N-O), 1342 (N-O).
1H NMR (500 MHz, CDCl3) δ 3.92 (s, 3H), 7.37 (m, 1H), 7.54 (t, J = 7.9 Hz, 2H), 7.83 (d, J
= 8.8 Hz, 4H), 8.24 (s, 1H), 8.39 (d, J = 8.9 Hz, 2H), 9.16 (s, 1H).
13C NMR (126 MHz, CDCl3) δ 53.3, 100.9, 115.1, 116.2, 120.0, 124.2, 128.7, 129.7,
129.8(2), 129.9, 137.2, 138.6, 144.9, 148.3, 153.6, 162.8.
MS (m/z): (M+H) Calcd for C21H17N4O4: 375.11. Found 375.20.
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Anal. Calcd. for C20H14N4O4: C, 64.17; H, 3.77; N, 14.97. Found: C, 64.12; H, 3.70; N,
14.92.
2.3.4. 2-(Benzo[d]thiazol-2-yl)-3-(1,3-diphenyl-1H-pyrazol-4-yl)acrylonitrile (3c)
3-(4-Nitrophenyl)-1-phenyl-1H-pyrazole-4-carbaldehyde (293 mg,
1
mmol) and 2-
benzo[d]thiazol-2-yl)acetonitrile (174 mg, 1 mmol) were dissolved in dry methanol (15 mL).
0.1 mL of piperidine were added, and the solution was stirred at RT for 2 h. After completion
of the reaction, the light yellow solid was filtered, washed with methanol and dried. The dye
obtained was purified by column chromatography using silica gel 100-200 mesh and toluene
as eluent system.
Color: Light yellow solid, Yield: 386 mg, 86%, M. P.: 284 – 285 oC;
FT-IR (KBr, cm-1): 2218 (CN), 1600 ( aromatic, C=C), 1529 (N-O), 1342 (N-O);
1H NMR (500 MHz, CDCl3) δ 7.44 - 7.45 (m, 2H), 7.52 - 7.58 (m, 3H), 7.89 (d, J = 7.8 Hz,
4H), 8.07 (s, 1H), 8.20 – 8.15 (m, 2H), 8.43 - 8.45 (d, J = 8.1 Hz, 2H), 9.17 (s, 1H).
13C NMR could not be recorded because of solubility problems.
MS (m/z): (M+H) Calcd for C25H16N5O2S: 450.10 found 450.40 (M+H).
Anal. Calcd. for C25H15N5O2S: C, 66.80; H, 3.36; N, 15.58; S, 7.13. Found: C, 66.84; H,
3.37; N, 15.51; S, 7.10.
2.3.5. 4-((1,3-Dipheny-1H-pyrazole-4-yl)methylene)-3-methyl-1-phenyl-1H-pyrazol-5(4H)-
one (3d)
3-(4-Nitrophenyl)-1-phenyl-1H-pyrazole-4-carbaldehyde (293 mg, 1 mmol) and 3-methyl-1-
phenyl-1H-pyrazol-5(4H)-one (174 mg, 1 mmol) were dissolved in dry methanol (15 mL).
0.1 mL of piperidine were added, and the solution was reflux for 12h. After completion of the
reaction, the reaction mixture cooled to room temperature the orange solid was filtered,
washed with methanol and dried. The dye obtained was purified by column chromatography
using silica gel 100-200 mesh and toluene as eluent system.
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Color: Orange Solid, Yield: 350 mg, 78%, M. P.: 251 – 252 oC
FT-IR (KBr, cm-1): 1674 (amide), 1597 (aromatic, C=C), 1500 (N-O), 1338 (N-O);
1H NMR (500 MHz, CDCl3) δ 2.26 (s, 3H), 7.21 (t, J = 7.3 Hz, 1H), 7.35 (s, 1H), 7.438-7.47
(m, 3H), 7.53 (t, J = 7.8 Hz, 2H), 7.89 (m, 4H), 7.96 (d, J = 7.9 Hz, 2H), 8.41 (d, J = 8.6 Hz,
2H), 10.27 (s, 1H).
13C NMR (126 MHz, CDCl3) δ 12.9, 115.9, 119.2, 119.9, 124.1, 124.8, 125.0, 128.2, 128.9,
129.7, 130.1, 133.5, 134.4, 137.9, 138.3, 138.8, 148.2, 150.0, 154.4, 162.4.
MS (m/z): (M+H) Calcd for C26H20N5O3: 450.15 found 450.40.
Anal. Calcd. for C26H19N5O3: C, 69.48; H, 4.26; N, 15.58. Found: C, 69.43; H, 4.20; N,
15.56.
2.3.6. 2-(3-(1,3-Dipheny-1H-pyrazole-4-yl)-1-phenylallylidene)malononitrile (3e)
3-(4-Nitrophenyl)-1-phenyl-1H-pyrazole-4-carbaldehyde (293 mg, 1 mmol) and 2-(1-
phenylallylidene)malononitrile (168 mg, 1mmol) were dissolved in dry methanol (15 mL).
0.1 mL of piperidine were added, and the solution was reflux for 6h. After completion of the
reaction, the reaction mixture cooled to room temperature the bright yellow solid was filtered,
washed with methanol and dried. The dye obtained was purified by column chromatography
using silica gel 100-200 mesh and toluene as eluent system.
Color: Yellow solid, Yield: 381 mg, 86%, M. P.: 230 – 232 oC.
FT-IR (KBr, cm-1): 2224 (cyano), 1514 (N-O), 1342 (N-O);
1H NMR (500 MHz, CDCl3) δ 6.90 (d, J = 15.5 Hz, 1H), 7.36 – 7.40 (m, 2H), 7.41-7.44 (m,
1H), 7.46 (d, J = 15.5 Hz, 1H), 7.52-7.61 (m, 5H), 7.66 (d, J = 8.9 Hz, 2H), 7.81 (dd, J = 8.6,
1.1 Hz, 2H), 8.23 (d, J = 8.9 Hz, 2H), 8.47 (s, 1H).
13C NMR (126 MHz, CDCl3) δ 81.5, 112.9, 113.3, 118.6, 119.6, 123.9, 124.8, 127.3, 128.2,
128. 7, 129.1, 129.8, 131.4, 132.8, 138.0, 138.6, 147.9, 151.3, 171.0.
MS (m/z): (M+H) Calcd for C27H18N5O2: 444.14 found 444.40.
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Anal. Calcd. for C27H17N5O2: C, 73.13; H, 3.86; N, 15.79. Found: C, 73.10; H, 3.82; N,
15.77.
2.3.7. 2-(3-(2(1,3-Dipheny-1H-pyrazole-4-yl)vinyl)5,5-dimethylcyclohex-2-en-1-
ylidene)malononitrile (3f)
3-(4-Nitrophenyl)-1-phenyl-1H-pyrazole-4-carbaldehyde (293 mg, 1 mmol) and 2-(3,5,5-
trimethylcyclohex-2-enylidene)malononitrile (186 mg, 1 mmol) were dissolved in dry
methanol (15 mL). 0.1 mL of piperidine were added, and the solution was reflux for 8h. After
completion of the reaction, the reaction mixture cooled to room temperature the red solid was
filtered, washed with methanol and dried. The dye obtained was purified by column
chromatography using silica gel 100-200 mesh and toluene as eluent system.
Color: Red solid, Yield: 369 mg, 80%, M. P.: 276 – 278 oC.
FT-IR (KBr, cm-1): 2228 (cyano), 1516 (N-O), 1342 (N-O);
1H NMR (500 MHz, CDCl3) δ 1.09 (s, 6H), 2.41 (s, 2H), 2.61 (s, 2H), 6.81 (s, 1H), 6.91 (d, J
= 16.0 Hz, 1H), 7.01 (d, J = 16.0 Hz, 1H), 7.41 (t, J = 7.9 Hz, 1H), 7.54 (t, J = 7.6 Hz, 2H),
7.80 (d, J = 8.5 Hz, 2H), 7.88 (d, J = 8.9 Hz, 2H), 8.32 (s, 1H), 8.39 (d, J = 8.8 Hz, 2H).
13C NMR (126 MHz, CDCl3) δ 28.0, 32.1, 38.9, 42.9, 78.9, 112.7, 113.4, 119.4, 119.8, 124.1,
125.8, 125.9, 127.8, 129.1, 129.7, 130.3, 138.9, 139.1, 147.7, 150.2, 153.5, 169.0.
HRMS m/z [M + H]+ Calcd for C28H24N5O2: 462.1930. Found: 462.2018.
Anal. Calcd. for C28H23N5O2: C,72.87; H, 5.02; N, 15.17. Found: C,72.82; H, 5.01; N, 15.24.
3. Result and Discussion
3.1. Chemistry
Herein, we have designed and synthesized a few nitro pyrazole based dyes featuring D-π-A
molecular configuration. N-phenyl pyrazole bearing nitro group has unique function
properties, Because of electron donating nature of pyrazole, we have introduced series of
acceptor group on the pyrazole unit. These molecules behave as D-π-A type and hence it may
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help to increase the optical properties. Compounds 1 and 2 were synthesized by using the
reported method [32,33]. Malononitrile, methylcyanoacetate, isophorone, 2-aminothiophenol
were available commercially. Active methylenes were synthesized by using the reported
methods [34–37]. All the synthesized intermediates and final products were carefully purified
and fully characterized by IR, 1H NMR and 13C NMR, and mass spectroscopic technique.
Scheme 1. Synthesis of the dyes 3a-3f
3.2. Photophysical properties
3.2.1. Effect of solvent polarity on absorption spectra
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In order to reveal the effect of multi-heteroatoms in electron donor moiety on the electronic
structure of dipolar chromophores D-π-A, UV-Vis absorption spectra of the pyrazole
substituted dyes were recorded in a series of nonpolar to polar solvents so that the
solvatochromic behaviour of each chromophore could be investigated in a wide range of
dielectric environments. The UV-Vis absorption spectra are shown in Fig. 1 and the data
listed in Table 1. The dyes 3a-3f showed absorption bands in the range of 330 – 421 nm. The
introduction of the nitro substituent into the aromatic portion of 3-(4-nitrophenyl)-1-phenyl-
1H-pyrazole nucleus was expected to result in a bathochromic shift due to enhanced electron
withdrawing ability of the chromophore. We have used various active methylenes to tune the
absorption spectra. All the dyes exhibited a similar broad π-π intramolecular CT absorption
band in the UV-Vis region with a continuous red-shift of the maximum absorption (λmax) due
to the gradually increasing acceptor strength. Compared with the dye 3a, the λmax of the dyes
3e and 3f were shifted to a longer wavelength of 402 nm and 411 nm respectively in DMSO,
which may be attributed to the additional double bond in acceptor moiety. In case of dye 3c
the absorption intensity was very low in methanol and ethanol solvents. This is due to less
solubility of dye in respective solvent. The dyes 3a-3f do not show any positive or negative
solvotochromism as there was no linear trend observed from non-polar to polar solvents.
<< Please Insert: Fig. 1. Absorption spectra of dyes 3a-3f in solvent of varying polarities.>>
3.2.2. DFT Studies
3.2.2.1. Optimized Geometry
The ground state geometries of the dyes 3a-3f were optimized at B3LYP/6-31G(d) in various
solvent of varying polarities. The optimized geometries of the dyes 3a-3f in DMF solvent are
shown in Fig. 2. The optimized geometry of the dyes 3a-3f suggest that the two phenyl ring
present on pyrazole core are shown twist in dihedral angle. The phenyl ring present on
nitrogen of pyrazole core is twisted about 25.1o to 29.1o. Moreover the another phenyl ring
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present on pyrazole core shows twist in the range of 34.8o to 40.9o. This result suggest that
molecule possess twisted geometry.
<< Please Insert: Fig. 2. Optimized geometry of dyes 3a-3f in DMF solvent at the ground
state using B3LYP/6-31G(d). >>
3.2.2.2. Vertical Excitation Spectra
In order to more understanding of the absorption properties of the synthesized pyrazole D-
πbridge-A dyes, the ground state optimized geometries of the pyrazole dyes in solvents of
various polarities were subjected to TD at least first 20 states in order to get vertical
excitations using the B3LYP/6-31G(d) method and the results are collected in Table 1. The
results of Density Functional Theory (DFT) and Time-Dependent Density Functional Theory
(TD-DFT) also suggest there is less or negligible influence of the change in solvent polarity
on the absorption of the pyrazole dyes 3a-3f (Table 1). The vertical excitations are associated
with the HOMO or HOMO-1 to LUMO or LUMO+1 transitions and strong oscillator
strengths ranging from 0.2890 to 1.0813. Table 1 reveals that dye 3a shows two absorption
peak one in the range of 302-308 nm and other in 343-356 nm while computed vertical
excitation shows closer value 347-352 with the oscillator strengths 0.2890-0.3069. Similar
observation are found for the dyes 3c, 3e and 3f show two values which more closely match
with the experimental values with comparable oscillator strength. Moreover, the dyes 3b and
3d show single absorption wavelength closer to the experimental absorption. The dye 3b
shows 3-16 nm deviation compared to the experimental value. The dye 3d shows more
deviation of 30 to 46 nm.
<< Please Insert: Table 1 Photophysical data of the dyes 3a-3f in various solvents of
varying polarities.>>
3.2.2.3. Frontier molecular orbitals
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The absorption leading to the electronic excitation from the HOMO or HOMO-1 to the
LUMO orbital can constitute the charge transfer from the N-phenyl pyrazole donor unit to the
various acceptor units such as dicyanovinyl, carbomethoxy cyanovinyl, cyanobenzthiazolyl
vinylene etc. In case of the dyes 3a, 3b, 3e, 3f the HOMOs are mainly located on N-phenyl
pyrazole core and π bridge while the LUMOs are located on the acceptor unit like
cyanovinylene and nitro phenyl ring (Table 2). However, for the dye 3c the HOMO is mainly
located on cyanovinyl benzthiazolyl unit and LUMO is spread all over the molecule, while
for the dye 3e the HOMO is located on methyl pyrazole unit and the LUMO is located on N-
phenyl pyrazole unit and bridge bond.
<< Please Inset: Table 2 Frontier molecular orbitals of dyes 3a-3f in the ground state.>>
The HOMO and LUMO energy level diagram gives quantitative idea of electronic structure
and excitation properties. Fig. 3 reveals that the calculated HOMO-LUMO energy band gap
of the dyes 3a-3f are in the range of 2.95-3.64 eV in DMF solvent. As shown in Fig. 3 the
dye 3f shows lower energy gap as compared to the dyes 3a-3e while the dye 3a shows
highest band gap of 3.64 eV (Table S1). This is because of the fact that the dye 3f shows
more π conjugation as compared to the dye 3a. The results reveal that lengthening of the π-
systems and electron-donating and electron accepting ability of chromophore leads to
lowering the HOMO-LUMO energy band gap.
<< Please Insert: Fig. 3. HOMO (H) – LUMO (L) energy level diagram of dyes 3a-3f in
DMF solvent.>>
3.2.2.4. Non Linear Optical (NLO) property study by computational method.
The linear polarizability (α0), first hyperpolarizability (β0) and second hyperpolarizability (γ)
values calculated using B3LYP/6-31G(d) on the basis of the finite field approach [38]. The
results are gathered in Table 3. The detailed theory and essential formulae which describing
the calculation of α0, β0 and γ values are provided in supporting information. The schematic
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representation for the first hyperpolarizability (β0) and second hyperpolarizability (γ) values
in various solvents for the dye 3f are shown Fig. 4.
Herein, we have found out that all the dyes show lower value for α, β0 and γ in 1,4-dioxane
and higher value in DMSO. For example the dye 3f shows lower β0 value at 143.56 x 10-30
esu in 1,4-dioxane and higher at 256.38 x10-30 esu in DMSO. These values are 377 and 677
times greater than urea (0.38 x 10-30 esu) respectively. The computed γ value for the dye 3f in
1,4-dioxane is 811.56 and 1580.51 x 10-36 esu in DMSO. These values are 1193 and 2324
times greater than urea (0.68 x 10-36 esu). The trend in α, β0 and γ values for dyes 3a-3f are 3f
> 3c > 3e > 3d > 3a > 3b (Table 4). We have also compared the β0 value of dyes 3e and 3f
with known analogue and we found that these dyes are showing greater β0 value (Table 4)
<< Please Inset: Fig. 4. The static first (β0) and second (γ) hyperpolarizability of dye 3f in
various solvents in comparison with urea.>>
Theoretical calculations reproduce the expected optical behaviour in the studied set of
pyrazole dyes. Increasing the length of conjugation in the set of pyrazole dyes 3a-3f (Table
S1) is accompanied by the decrease in HOMO-LUMO band gap, along with increase in
oscillator strength that results into enhanced NLO response (larger α, β0 and γ values). The
hyperpolarizability values are markedly influenced by the solvent effects, from nonpolar to
polar solvents hyperpolarizability values showing increasing tendency.
<< Please Insert: Table 3 Theoretically calculated nonlinear optical properties of the dyes
3a-3f in various solvents at B3LYP level using 6-31G(d) basis set.>>
<< Please Insert: Table 4 Comparison of first hyperpolarizability values of dyes 3e and 3f
with known analogues. >>
4. Conclusion
A series of novel donor–π-bridge-acceptor pyrazole based dyes (3a-3f) have been synthesized
and characterized by spectral technique. The simple and easy access to pyrazole dyes 3a-3f
13

ACCEPTED MANUSCRIPT
were carried out through the combination of Vilsmeier–Haack formylation and Knoevenagel
condensation reaction. The synthesized dyes were optimized at B3LYP/6-31G(d) level in the
ground state geometry. The synthesized pyrazole dyes were thoroughly investigated for their
molecular nonlinear optical (NLO) properties at the theoretical level using density functional
theory methods. A pronounced conformational effect on calculated hyperpolarizabilities for
α, β0 and γ were found for pyrazole derivatives (3c, 3e and 3f). All the dyes under
investigation could be considered as efficient NLOphores due to their very large β0 and γ
values. Comparison of synthesized dyes with urea and known analogue showed the
advantage in terms of nonlinear optical properties. In general, it was shown that inclusion of a
π bridge-acceptor group on pyrazole ring substantially enhances the NLO response of the
donor-π -acceptor systems.
Acknowledgements
The authors are thankful to SAIF- Indian Institute of Technology, Bombay for recording the
mass spectra. Sandip K Lanke is thankful to UGC and CSIR, New Delhi for providing junior
and senior research fellowships.
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Tables All
List of Tables
1. Table 1 Photophysical data of the dyes 3a-3f in various solvents of varying polarities.
2. Table 2 Frontier molecular orbitals of dyes 3a-3f in the ground state.
3. Table 3 Theoretically calculated nonlinear optical properties of the dyes 3a-3f in
various solvent at B3LYP level using 6-31G(d) basis set.
4. Table 4 Comparison of first hyperpolarizability values of dyes 3e and 3f with known
analogues.
1

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Tables All
Table 1 Photophysical data of the dyes 3a-3f in various solvent of varying polarities.
Experimental
Theoretical [B3LYP/6-31G(d)]
Vertical
Excitation
aλabs
bε
dµge
(Debye)
Orbital
contribution(%)
Dye Solvent
cf
(nm) ( L mol−1 cm−1)
(nm)
eV
Toluene
Dioxane
THF
356,
308
346,
305
346,
306
352,
302
346,
303
345,
302
346,
302
347
23449, 22465
45272, 43806
30583, 29064
35434, 33212
40364, 38407
30917, 30306
38358, 37101
23617
377 3.2902 0.3790
347 3.5687 0.3016
376 3.3013 0.3716
347 3.5769 0.2899
382 3.2492 0.3345
351 3.5320 0.2973
380 3.2601 0.3502
350 3.5404 0.3006
381 3.2580 0.3334
350 3.5383 0.2906
383 3.2343 0.3082
352 3.5239 0.2890
383 3.2336 0.3147
352 3.5235 0.2944
384 3.2252 0.3229
352 3.5177 0.3069
384 3.2251 0.3201
352 3.5180 0.3050
5.52
4.72
5.46
4.63
5.22
4.71
5.32
4.73
5.20
4.65
5.01
4.65
5.07
4.70
5.14
4.80
5.12
4.78
H→L 98.7
H→L+1 90.2
H→L 98.7
H→L+1 92.0
H→L 99.1
H→L+1 91.5
H→L 98.9
H→L+1 92.0
H→L 99.0
H→L+1 91.3
H→L 99.2
H→L+1 90.3
H→L 99.2
H→L+1 90.7
H→L 99.1
H→L+1 91.1
H→L 99.2
CHCl3
EtOAc
3a
MeOH
EtOH
DMF
DMSO
343,
306
32150, 30314
H→L+1 91.1
Toluene
Dioxane
THF
347
333
333
339
330
330
335
330
330
21747
25829
27527
14013
28638
34027
35164
33024
28835
344 3.6068 0.3796
343 3.6143 0.3678
345 3.5923 0.3521
345 3.5953 0.3624
345 3.5972 0.3470
345 3.5907 0.3333
345 3.5880 0.3403
346 3.5827 0.3533
346 3.5834 0.3506
5.27
5.18
5.09
5.16
5.05
4.95
5.00
5.10
5.08
H→L+1 91.3
H→L+1 90.8
H→L+1 89.6
H→L+1 90.3
H→L+1 89.5
H→L+1 88.2
H→L+1 88.6
H→L+1 89.2
H→L+1 89.1
CHCl3
EtOAc
MeOH
EtOH
3b
DMF
DMSO
427 2.9046 1.0035
369 3.3564 0.1378
425 2.9152 0.9862
369 3.3621 0.1211
430 2.8828 0.9243
372 3.3293 0.2104
429 2.8874 0.9539
371 3.3364 0.1893
429 2.8903 0.9207
372 3.3347 0.1951
431 2.8778 0.8714
373 3.3205 0.2392
431 2.8755 0.8856
373 3.3199 0.2392
432 2.8663 0.9047
374 3.3145 0.2507
432 2.8669 0.8989
374 3.3145 0.2514
9.55
3.29
9.45
3.09
9.20
4.08
9.34
3.87
9.17
3.93
8.94
4.36
9.02
4.36
9.12
4.47
9.09
4.48
H→L 97.5
H→L+1 64.6
H→L 97.6
H→L+1 60.4
H→L 97.7
H→L+1 71.1
H→L 97.6
H L+1 70.0
H→L 97.7
H→L+1 69.0
H→L 97.9
H→L+1 70.8
H→L 97.8
H→L+1 71.7
H→L 97.6
H→L+1 73.7
H→L 97.7
Toluene
Dioxane
THF
383
379
378
382
375
-
13141
21052
23736
32049
35234
-
CHCl3
EtOAc
MeOH
EtOH
3c
-
-
DMF
376
375
24370
12448
DMSO
H→L+1 73.4
Toluene
Dioxane
THF
364
361
343
27277
35120
37978
394 3.1506 0.4396
392 3.1586 0.4314
396 3.1283 0.3887
6.07
6.00
5.73
H-1→L 95.9
H-1→L 96.2
H-1→L 94.9
2

ACCEPTED MANUSCRIPT
Tables All
3d
CHCl3
EtOAc
MeOH
EtOH
363
359
360
362
361
364
42436
39127
33560
8626
396 3.1326 0.4046
396 3.1332 0.3870
397 3.1265 0.3638
396 3.1246 0.3705
397 3.1195 0.3801
397 3.1203 0.3772
5.84
5.71
5.55
5.59
5.67
5.65
H-1→L 95.3
H-1→L 95.3
H-1→L 94.3
H-1→L 94.3
H-1→L 93.9
H-1→L 93.9
DMF
38657
43969
DMSO
421 0.7362 0.7362
386 0.2447 0.2447
419 2.9602 0.7194
385 3.2214 0.2384
426 2.9077 0.6480
392 3.1589 0.2823
425 2.9180 0.6808
391 3.1742 0.2687
425 2.9184 0.6454
391 3.1680 0.2740
428 2.8936 0.5881
395 3.1384 0.3078
429 2.8919 0.6035
395 3.1387 0.3060
430 2.8805 0.6229
396 3.1312 0.3130
430 2.8807 0.6167
396 3.1307 0.3139
8.13
4.49
8.01
4.42
7.67
4.85
7.85
4.73
7.64
4.78
7.32
5.09
7.43
5.07
7.55
5.14
7.52
5.15
H→L 94.3
H→L+1 91.2
H→L 93.7
H→L+1 90.5
H→L 94.4
H→L+1 89.4
H→L 94.4
H→L+1 90.2
H→L 93.9
H→L+1 89.1
H→L 94.1
H→L+1 87.7
H→L 94.3
H→L+1 88.4
H→L 94.9
H→L+1 89.4
H→L 94.9
Toluene
Dioxane
THF
396
391
394
397
390
391
393
399
402
37341
41651
25983
39784
43665
48224
38250
20475
23906
CHCl3
EtOAc
MeOH
EtOH
3e
DMF
DMSO
H→L+1 89.2
454 2.7314 1.0813
417 2.9677 0.3638
451 2.7472 1.0535
416 2.9789 0.3677
462 2.6817 0.8589
426 2.9079 0.5543
460 2.6960 0.9410
424 2.9232 0.4854
460 2.6945 0.8599
425 2.9189 0.5403
466 2.6593 0.7169
429 2.8907 0.6710
466 2.6587 0.7496
429 2.8891 0.6509
468 2.6462 0.7856
431 2.8778 0.6417
469 2.6458 0.7715
431 2.8778 0.6514
10.23
5.68
10.06
5.71
9.91
7.09
9.60
6.62
9.18
6.99
8.44
7.63
8.63
7.71
8.85
7.68
8.78
7.73
H→L 92.5
H→L+1 91.6
H→L 91.5
H→L+1 90.5
H→L 89.8
H→L+1 87.6
H→L 90.9
H→L+1 89.9
H→L 89.2
H→L+1 86.9
H→L 87.9
H→L+1 84.8
H→L 88.8
H→L+1 85.8
H→L 90.5
H→L+1 87.7
H→L 90.2
Toluene
Dioxane
THF
413
411
414
415
410
411
415
421
422
35312
21938
19525
32588
34314
31073
31238
61108
33001
CHCl3
EtOAc
MeOH
EtOH
3f
DMF
DMSO
H→L+1 87.4
aλabs = absorption wavelength maxima (nm), bε = molar extinction coefficient at conc. 25 x 10-6 mol L-1, cf =
oscillator strength, dµge = transition dipole moment.
3

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Tables All
Table 2 Frontier molecular orbitals of dyes 3a-3f in the ground state.
Dye
HOMO-1
HOMO
LUMO
LUMO+1
3a
3b
3c
3d
3e
3f
4

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Tables All
Table 3 Theoretically calculated nonlinear optical properties of the dyes 3a-3f in various
solvents at B3LYP level using 6-31G(d) basis set.
b
d
ꢀa
Debye
α0
ꢁα c
β0
γe
ꢀβ0
Dye
3a
Solvent
10-24(esu)
10-24 (esu)
10-30 (esu)
10-36 (esu)
10-48 (esu)
Toluene
THF
Dioxane
EA
DCM
CHCl3
MeOH
EtOH
DMF
8.7
9.3
8.6
9.2
9.3
9.1
9.5
9.5
9.5
9.5
45.28
49.99
44.87
49.32
50.46
48.52
52.28
52.05
52.37
52.51
3351.37
4027.08
3289.31
3948.96
4089.10
3824.69
4315.80
4289.42
4326.22
4341.23
50.64
72.61
50.20
69.74
74.74
65.97
83.13
82.08
83.55
84.16
235.71
347.48
226.94
331.31
359.62
310.38
408.52
402.31
411.01
414.67
440.57
675.27
431.72
641.61
695.08
600.33
789.74
779.76
793.73
799.52
DMSO
Toluene
THF
Dioxane
EA
DCM
CHCl3
MeOH
EtOH
DMF
3.7
4.1
3.7
4.0
4.1
4.0
4.2
4.2
4.2
4.2
48.99
53.85
48.57
53.19
54.34
52.32
56.26
56.02
56.36
56.50
3434.43
4049.60
3374.77
3972.97
4104.74
3867.01
4303.42
4280.37
4312.49
4325.64
54.83
73.35
53.24
70.79
75.24
67.43
82.67
81.74
83.04
83.59
236.66
338.98
228.42
324.17
350.06
305.05
394.86
389.14
397.16
400.54
202.87
300.74
196.99
283.16
308.48
269.72
347.21
343.31
348.77
351.08
3b
DMSO
Toluene
THF
Dioxane
EA
DCM
CHCl3
MeOH
EtOH
DMF
8.8
9.5
8.8
9.4
9.5
9.3
9.8
9.8
9.8
9.8
66.83
73.85
66.21
72.91
74.55
71.67
77.28
76.93
77.42
77.62
11636.57
13929.88
11421.32
13642.98
14135.79
13255.12
14894.37
14802.90
14930.38
14983.01
116.93
153.96
113.67
149.08
157.51
142.69
171.11
169.42
171.79
172.77
919.44
1328.59
886.30
1299.47
1371.49
1196.21
1542.86
1521.05
1551.62
1564.49
1028.98
1462.62
1000.30
1401.35
1496.35
1327.02
1676.88
1660.32
1683.54
1693.15
3c
DMSO
Toluene
THF
Dioxane
EA
DCM
CHCl3
MeOH
EtOH
DMF
4.5
4.7
4.5
4.7
4.7
4.6
4.7
4.7
4.7
4.7
62.23
68.40
61.68
67.56
69.01
66.47
71.40
71.10
71.53
71.70
6006.76
6888.79
5921.03
6784.39
6961.35
6638.79
7204.72
7179.89
7217.91
7228.79
63.01
84.52
61.17
81.59
86.66
77.74
94.97
93.95
95.42
95.99
365.24
516.81
352.87
495.43
532.60
467.67
595.17
587.36
598.59
602.98
283.55
397.24
275.27
383.47
407.30
357.60
446.36
441.57
448.47
451.15
3d
DMSO
Toluene
THF
Dioxane
EA
DCM
CHCl3
MeOH
EtOH
7.6
8.5
7.6
8.5
8.6
8.2
8.9
8.8
65.37
72.97
64.71
71.92
73.74
70.55
76.83
76.44
5391.30
6687.12
5277.80
6513.40
6813.80
6283.80
7304.39
7243.44
90.76
130.47
87.59
124.74
134.75
117.37
152.14
149.90
468.97
711.07
450.63
674.72
738.46
628.69
852.32
837.46
689.78
1109.00
665.68
1060.29
1158.85
962.43
3e
1354.05
1319.12
5

ACCEPTED MANUSCRIPT
Tables All
DMF
DMSO
8.9
8.9
76.99
77.22
7328.77
7364.45
153.04
154.37
858.33
867.27
1362.06
1373.89
Toluene
THF
Dioxane
EA
DCM
CHCl3
MeOH
EtOH
DMF
8.2
9.0
8.1
8.9
9.0
8.7
9.3
9.3
9.3
9.3
74.80
83.74
74.03
82.51
84.65
80.90
88.28
87.82
88.47
88.74
9968.58
12647.35
9744.61
12275.14
12921.19
11789.45
13997.95
13861.89
14051.84
14130.71
148.91
215.97
143.56
206.27
223.24
193.82
252.61
248.81
254.14
256.38
845.16
1291.38
811.56
1224.15
1342.08
1139.14
1553.02
1525.29
1564.13
1580.51
1221.06
1943.73
1162.84
1835.80
2009.16
1686.23
2349.27
2313.93
2363.50
2384.33
3f
DMSO
aStatic dipole moment (ꢀ), bmean polarizability (α0), cpolarizability anisotropy (ꢁα), dfirst hyperpolarizability
(β0) and esecond hyperpolarizability (γ).
Table 4 Comparison of first hyperpolarizability values of dyes 3e and 3f with known
analogues.
Sr. No.
Dye
β
µβ
Reference
1
65
689
[39]
2
3
138
65
1642
540
[39]
[39]
4
196
867
-
[40]
O2N
CN
CN
3e
1373
This work
N
N
CN
CN
O2N
3f
1580
2384
This work
N
N
6

ACCEPTED MANUSCRIPT
All Figures
List of Figures
1. Fig. 1. Absorption spectra of dyes 3a-3f in solvent of varying polarities.
2. Fig. 2. Optimized geometry of dyes 3a-3f in DMF solvent at the ground state using
B3LYP/6-31G(d).
3. Fig. 3. HOMO (H) – LUMO (L) energy level diagram of dyes 3a-3f in DMF solvent.
4. Fig. 4. The static first (β0) and second (γ) hyperpolarizability of dye 3f in various solvents in
comparison with urea.
1

ACCEPTED MANUSCRIPT
All Figures
DMF
1.0
DMF
3a
3b
1.2
1.0
0.8
0.6
0.4
0.2
0.0
DMSO
EtOH
MeOH
EA
DMSO
EtOH
0.8
MeOH
EA
CHCl3
THF
Dioxane
Toluene
CHCl3
THF
Dioxane
Toluene
0.6
0.4
0.2
0.0
290 310 330 350 370 390
300 320 340 360 380 400
Wavelength (nm)
Wavelength (nm)
DMF
DMSO
EtOH
MeOH
EA
CHCl3
THF
DMF
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
3c
3d
DMSO
EtOH
MeOH
EA
CHCl3
THF
Dioxane
Toluene
Dioxane
Toluene
300
330
360
390
420
450
300 330 360 390 420 450
Wavelength (nm)
Wavelength (nm)
DMF
DMF
DMSO
EtOH
MeOH
EA
CHCl3
THF
1.4
1.2
3e
3f
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
DMSO
EtOH
MeOH
EA
CHCl3
THF
1.0
0.8
0.6
0.4
0.2
0.0
Dioxane
Toluene
Dioxane
Toluene
350 370 390 410 430 450 470
290
340
390
440
490
Wavelength (nm)
Wavelength (nm)
Fig. 1. Absorption spectra of dyes 3a-3f in solvent of varying polarities.
2

ACCEPTED MANUSCRIPT
All Figures
3

ACCEPTED MANUSCRIPT
All Figures
Fig. 2. Optimized geometry of dyes 3a-3f in DMF solvent at the ground state using B3LYP/6-
31G(d).
Fig. 3. HOMO (H) – LUMO (L) energy level diagram of dyes 3a-3f in DMF solvent.
4

ACCEPTED MANUSCRIPT
All Figures
β x 10E-30 (esu
γ x 10E-36 (esu)
1600.00
1400.00
1200.00
1000.00
800.00
600.00
400.00
200.00
0.00
Fig. 4. The static first (β0) and second (γ) hyperpolarizability of dye 3f in various solvents in
comparison with urea.
5